I have also encountered this sort of exception when the calendar was working on the main thread:
at (wrapper managed-to-native) UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
at UIKit.UIApplication.Main (System.String[] args, System.IntPtr principal, System.IntPtr delegate) [0x00005] in /Users/builder/data/lanes/3859/b638977c/source/xamarin-macios/src/UIKit/UIApplication.cs:79
at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x00038] in /Users/builder/data/lanes/3859/b638977c/source/xamarin-macios/src/UIKit/UIApplication.cs:63
at IQOSMobile.iOS.Application.Main (System.String[] args) [0x00001] in Main.cs